Federal and D.C. employees may–

• be candidates for public office in nonpartisan elections
• register and vote as they choose
• assist in voter registration drives
• express opinions about candidates and issues
• contribute money to political organizations
• attend political fundraising functions
• attend and be active at political rallies and meetings
• join and be an active member of a political party or club
• sign nominating petitions
• campaign for or against referendum questions, constitutional amendments, municipal ordinances
• campaign for or against candidates in partisan elections
• make campaign speeches for candidates in partisan elections
• distribute campaign literature in partisan elections
• hold office in political clubs or parties

Federal and D.C. employees may not–

• use official authority or influence to interfere with an election
• solicit or discourage political activity of anyone with business before their agency
• solicit or receive political contributions (may be done in certain limited situations by federal labor or other employee organizations)
• be candidates for public office in partisan elections
• engage in political activity while:
  • on duty
  • in a government office
  • wearing an official uniform
  • using a government vehicle
• wear partisan political buttons on duty

http://www.osc.gov/ha_fed.htm